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Yogurt Connoisseur Wiki
What is Yogurt? Yogurt is a milk product that is produced through bacterial fermentation. The name, yogurt, itself is a broad classification of several milk products that share a similar preliminary step of bacterial fermentation. From here, several techniques are used to modify the fermented milk product for different culinary applications. Furthermore, certain types of yogurt is claimed to have several positive effects on overall health through a variety of research methodologies . __TOC__ How is it made? In most cases, cow's milk is used. However, this can be substituted for any mammalian milk sources. Since fat in raw milk normally settles on the top, the milk is first homogenized at high pressures to emulsify fat globules evenly throughout the solution. Milk is then pasteurized to prevent contaminant bacterial growth. The next step involves innoculating milk with a bacterial culture and incubated at around 42 degrees Celsius until the product is firm. This change in viscosity occurs due to lactate production during anaerobic fermentation. Yogurt texture is only one characteristic that plays into yogurts entire sensory evaluation of taste, acidity, smell and appearance. (If you're interested in producing yogurt at home, simply purchase milk at your grocery store and you're ready to start. Chances are that it is already homogenized and pasteurized.) Blah Blah Blah Testing Research Methodologies The sensory evaluation of the final product should be prioritized over any other type of analysis. From a technical standpoint, the researchers could only do a sensory evaluation, as there are no standards for testing the physical properties of the yogurt. The researchers were attempting to study the effect of a LT-ROF production on the physical, sensory, properties, such as taste and consistency of the yogurt. Ultimately, the researchers are testing a new production method for yogurt, and if the yogurt is not pleasing to one’s palate, the product will not sell, and the experimental production method is a failure regardless of what any data may say. The fact that the researchers used a large pool of consumers who regularly consume yogurt is a major positive, as the sensory evaluation from these individuals will be well-informed due to the individuals’ love and experience with yogurt. Thus, even though a sensory evaluation is subjective, it is the most important form of evaluation in any study regarding the production of foods. A bad tasting product will simply not sell, dooming it to market failure and thus the failure of the study's ultimate goal. Importance of Bacterial Metabolism to Yogurt Production Both S. thermophilus and L. bulgaricus are facultative anaerobes, meaning that they have the ability to oxidize glucose using molecular oxygen if it is available''Zourari, A., Accolas, J. P., and Desmazeaud, M. J. (1992) Metabolism and biochemical characteristics of yogurt bacteria. A review. Le Lait 72, 1–34'. 'However, the conditions under which yogurt is made are generally anaerobic, requiring that both species shift their metabolism towards fermentation. In fact, both species grow best in anaerobic conditions, in part due to their inability to handle reactive oxygen species generated by aerobic metabolism'Higuchi, M., Yamamoto, Y., and Kamio, Y. (2000) Molecular biology of oxygen tolerance in lactic acid bacteria: Functions of NADH oxidases and Dpr in oxidative stress. Journal of Bioscience and Bioengineering 90, 484–493Condon, S. (1987) Responses of lactic acid bacteria to oxygen. FEMS Microbiology Letters 46, 269–280'. The role of starter cultures in yogurt production is primarily to acidify milk by converting lactose into lactic acid'''Lee, W. J., and Lucey, J. A. (2010) Formation and physical properties of yogurt. Asian-Aust. J. Anim. Sci 23, 1127–1136'. The disaccharide lactose can be degraded to glucose and galactose, with the latter sugar entering glycolysis via the Leloir pathway. Carbon shuttled into glycolysis ultimately finds itself in the form of pyruvate. The organism uses the free energy difference between glucose and pyruvate to synthesize ATP and reduce NAD+ to NADH. In the absence of molecular oxygen as a final electron acceptor, the electron transport chain is unable to reoxidize NADH to NAD+, upon which glycolyis is dependent. To ensure the regeneration of NAD+ and the continual production of ATP from glycolysis, pyruvate is reduced to lactic acid using NADH as an electron donor. The lactic acid produced by anaerobic fermentation reduces the pH of the milk. Depending on the nature of the heat treatment, the milk undergoes gelation around pH 5.3, creating a firmer texture'''Lee, W. J., and Lucey, J. A. (2010) Formation and physical properties of yogurt. Asian-Aust. J. Anim. Sci 23, 1127–1136''. Role of yogurt bacteria The two primary organisms that are responsible for lactic acid production in yogurt are Streptococcus thermophilus and Lactobacillus delbrueckii subsp bulgaricus. Lactic acid is able to acidify milk, providing its sharp, acidic taste and flavour. Both bacteria are facultative anaerobes, meaning that they are able to make ATP by both aerobic respiration and fermentation. Streptococcus thermophiles and Lactobacillus delbrueckii subsp bulgaricus synergistically work together to stimulate their growth and acid production. They also work together to stimulate production of metabolites such as acetaldehyde. These two strains are able to work together by exchanging metabolites such as folic acid and carbon dioxide to supplement for the absence of various amino acids. A study by Sieuwerts et al. on the biochemical interactions between the two bacteria performed mixed-culture transcriptome profiling and a systemic analysis on the effect of interaction-related compounds of growth. In L bulgaricus, t was found that the genes associated with purine metabolism are downregulated and stu0036, a permease of xaine/uracil is downregulated to provide purine precursors for ''S. thermophilus. ''Furthermore, there was an upregulation of genes for the biosynthesis of arginine and branched chain amino acids. Notes